Take a look at your left thumb. Now, look at your right thumb. Are they identical (that is, are they exactly the same)?

Of course not! The two thumbs look quite similar, but, even without a microscope, we can say that they don't look the same. In fact, almost no object you see in your daily life is identical to a different object.

Yet, when we say Water is made up of Hydrogen and Oxygen, are all Oxygen atoms the same? If I see an Oxygen atom in Kohlapur, is it identical to an Oxygen atom in Kohima? Let's explore this question in this module.